This invention relates generally to guided wave acousto-optic devices and particularly to such a device suitable to provide a correlation function, Fourier transform or convolution integral.
In up-to-date communication systems spread spectrum techniques are used because they have various advantages. They are less susceptible to interfering signals, provide better signal-to-noise ratios and have processing gain and better time revolution. Among these spread spectrum techniques are spread spectrum codes which may have a length of 500 bits. Such a group code is usually a pseudo random code.
In the past it has been proposed to process such codes by digital, large-scale integration (LSI) structures. Also, programable tapped surface acoustic wave delay lines have been proposed for this purpose. However, such digital LSI devices handling code lengths of 500 bits are limited to bit rates of a few megabits per second and have a rather high power consumption. On the other hand, the programable acoustic wave filters have high bit rates which may be as high as 10 megabits per second and can receive code sequence lengths of over 100 bits. However, they are difficult to manufacture because a large number of connections must be made in a very limited space.
Various optical correlators have been proposed in the past. For example, a paper by Slobodin which appears on page 1782 of Proceedings of the IEEE, December 1963, discloses an optical correlator. A light beam is sent through an acoustic wave generated in a liquid by a quartz transducer. The output is collected by a phototube. The thus generated acoustic wave is compared to a film replica disposed in the liquid.
A similar system has been proposed in a paper by Atzeni et al. which appears on pages 501 to 502 of Proceedings of the IEEE, March 1970. It has also been suggested to provide a correlation detector by sending the light beam through two spaced acoustic columns travelling in opposite directions which may, for example, be generated in two sapphire rods. This has been discussed in a paper by Squire et al. which was published by the Naval Undersea Warfare Center in May 1968. A similar system has been proposed in a paper by Montgomery et al. which appears in the IEEE 1973 Ultrasonic Symposium Proceedings on pages 553-557.
Surface wave filters in general have been discussed in a paper by Squire et al. which appears in IEEE Transactions, Volume MTT-17, No. 11, November 1969, pages 1020 - 1040. The surface wave filters are explained on page 1034 et seq.
An optical guided wave and its interaction with a surface acoustic wave has been discussed in a paper by Kuhn et al. which appears in Applied Physics Letters, Volume 17, No. 6 of Sept. 15, 1970, pages 265 to 267. The surface acoustic wave is propagated along a quartz crystal. A thin glass film is deposited on top of the crystal and serves as an optical wave guide. The light beam is coupled to the glass film by a grating coupler. The optical guided wave is then deflected by the surface acoustic wave. In this case, however, there is only a single acoustic wave and the light beam must be specially coupled to the surface layer.
Guided optical waves in lithium niobate have been disclosed in a paper by Schmidt et al. which appears in Applied Physics Letters, Volume 23, No. 8, Oct. 15, 1973, pages 417 - 419. A surface acoustic wave is launched in the lithium niobate crystal. The crystal is outdiffused to provide a surface layer with increased index of refraction. The light wave is launched into the outdiffused surface layer by rutile prism couplers.
Also, real time convolution of two signals has been obtained in a setup described in a paper by Kramer et al. which appears in Applied Physics Letters, Volume 25, No. 4 Aug. 15, 1974, pages 180 - 183. Here two surface acoustic waves are launched in a lithium niobate (LiNbO.sub.3) delay line. However, the light beam is sent through the polished side wall into the center of the crystal. Thus the light beam is not a guided optical beam.
A recent paper to Tsai et al. which appears in Applied Physics Letters, Volume 26, No. 4 of Feb. 15, 1975, pages 140 - 142 relates to an acousto-optica guided light-beam device utilizing two tilting surface acoustic-waves. The device may be used as a modulator, switch, or deflector and utilizing an outdiffused surface layer of a lithium niobate plate to guide a light beam. The light beam is coupled into the surface layer by an input prism and a similar prism is used to couple the light beam out of the device. The two acoustic waves are tilted with respect to each other. No reference is made to a correlator nor would the device function as such.
It is accordingly an object of the present invention to provide an acousto-optic device suitable for obtaining the correlation function, Fourier transform or convolution integral of two signals which is characterized by a bit rate greater than 10 megabits per second and capable of handling more than 500 bits of code length.
It is accordingly an object of the present invention to provide such a device capable of operating within an acoustic frequency range of approximately 100 megahertz (mhz) to approximately one gigahertz (ghz).
Another object of the present invention is to provide a device of the character disclosed which has a surface layer having a thickness on the order of 50 micrometers (0.05 millimeter) and which is capable of interacting with great efficiency with a surface acoustic wave, thereby to improve the signal-to-noise ratio.